This invention relates to an improved process for delignifying and brightening lignocellulosic pulp. More particularly, the invention is directed at a peroxide-based delignifying and bleaching solution and process for lignocellulosic pulp. In this process a lignocellulosic pulp is contacted at a temperature below about 50.degree. C. with a solution of metal ions selected from the group consisting of tungsten, molybdenum, chromium, osmium and selenium. The pH of the resultant slurry is adjusted to between about 1-7. A peroxide is added and pulp delignification and brightening effected at 30.degree.-100.degree. C. until most of the peroxide has been consumed. The process affords pulp delignification and brightening with minimal destruction of the cellulosic portion of the pulp.
Wood is composed of two main parts--a fibrous carbohydrate or cellulosic portion and a non-fibrous portion comprising a complex chemical, commonly referred to as lignin.
For use in paper-making processes, wood must first be reduced to pulp, which can be defined as wood fibers capable of being slurried or suspended and then deposited as a screen to form a sheet. The methods employed to accomplish this pulping usually involve either physical or chemical treatment of the wood or some combination of the two processes to alter its chemical form and to give desired paper properties.
In mechanical pulping, the wood is physically ground to a high-yield, lignin retained pulp, most often referred to as groundwood pulp. In chemical pulping, the wood chips are digested with a chemical solution to solubilize a portion of the lignin and effect its removal. The more usual of these digestive procedures are the sulfite, sulfate or Kraft, soda and modified sulfite processes.
After the wood has been digested or mechanically ground, the resulting material is generally a darkly colored cellulosic fiber. The dark color is attributable to the fact that not all of the lignin has been removed during digestion and none has been removed during mechanical processing. This dark pulp is commonly referred to as unbleached pulp. Unbleached pulp may pass directly to the paper making operation only if paper color is unimportant.
More usually, unbleached lignocellulosic pulps are bleached or brightened to a brightness consistent with the planned utilization of the pulp, brightness being a measure of pulp reflectivity under standardized conditions. Pulp bleaching is most often a multi-stage process employing various chemicals to remove or alter the lignin of the lignocellulosic pulp such that the resultant pulp is no longer light absorbing or dark in color.
Two classes of compounds generally find use as lignocellulosic brighteners, namely reducing agents and oxidizing agents. Common reducing agents include sulfurous acids, hydrosulphites, borohydrides, amine boranes, and bisulfites. Common oxidizing agents include chlorine-based compounds, peroxides, ozone, oxygen, peracids, permanganates and chromates.
The ready availability and environmentally appealing character of peroxides have made them increasingly popular in recent years as bleachants in the paper-making industry. Such peroxides are used in the "lignin retaining" bleaching of groundwood and other high yield pulps to provide substantial brightness gains, but not the high brightness attainable with chemical pulps. Such processes are described, for example, in Hook, "Peroxide Bleaching Opens New Potentials For Groundwood Pulps", Pulp & Paper International, 45-48 (June 1975), Vartiainen, "Utilization of Peroxide in Pulp Bleaching", Papper Och Tra, 51, 277-284 (1969), U.S. Pat. Nos. 2,187,016, 3,023,140, and 2,251,731 and Canadian Pat. No. 970,111. Additionally, peroxides are employed as a "capping" or super-bleach stage at the end of common multi-stage bleaching processes for chemical pulps. Exemplifying these processes are those described in Canadian Pat. Nos. 966,604 and 970,111, U.S. Pat. Nos. 3,193,445 and 3,462,344 and Vartiainen, "Utilization of Peroxide in Pulp Bleaching", Papper Och Tra, 51, 277-284 (1969), Delattre, "Hydrogen Peroxide as a Bleaching Agent for Kraft Pulps", Papper Och Tra, 117-127 (1971), Hartler et al., "Peroxide Bleaching of Kraft Pulps", Tappi, 43, 806-813 (1960), Christensen, "Bleaching Sulphate Pulp with Hydrogen Peroxide", Pulp and Paper Magazine of Canada, 62-66 (1971), Christensen, "Bleaching of Sulphate Pulps with Hydrogen Peroxide", Norsk Skogindustri, 268-271 (October, 1973), and Mlakar & Peltonen, "Peroxide in the Semi-bleaching of Kraft Pulp", Papper Och Tra, 11, 629-638 (1968).
However, peroxide bleaching by itself is not able to effect sufficient pulp bleaching for most paper-making requirements. This is particularly true when the peroxide is employed in an internal stage in a multi-stage bleaching process after initial ozone or oxygen bleaching. Previous variations of the conditions for peroxide-pulp contact have either not remedied this inferior bleaching characteristic or so degraded the cellulosic portion of the pulp that it was no longer useful for paper-making processes.
The failure of peroxide bleachants in internal or prebleaching stages is apparently due to the inability of peroxide to delignify lignocellulosic pulps sufficiently to remove the light-absorbing, dark lignin components and yet, maintain that resultant pulp viscosity and strength necessary for subsequent paper-making processes.
For example, at more alkaline pH's, i.e. 9-11, peroxide bleaching is usually conducted in the presence of sodium silicate and magnesium sulfate stabilizers to avoid peroxide decomposition by heavy metal contaminants. But, under such conditions substantially no delignification is obtained. At higher pH's auto-decomposition of the peroxide becomes excessive so as economically to disadvantage the process.
Conversely, at acid pH's, i.e. 2-7, the peroxide reaction with the lignin chromophores of the pulp is sluggish and often accompanied by severe visocisity losses. Rapson, "The Rule of pH in Bleaching Pulp", Tappi, 39, 284-294 (1956) and U.S. Pat. No. 2,112,116. Similar pulp viscosity and strength reductions are adduced by the addition of metals such as manganese, cobalt, iron, copper and aluminum to oxidizing bleaching agents such as peroxide. These special oxidants have been used to good advantage in U.S. Pat. Nos. 2,975,169 and 2,368,527 to produce pulp, which while unsuitable for paper manufacture, is well constituted for the production of viscose rayon or other cellulose-like derivatives.
Other additives are likewise ineffective in improving the brightness gain achievable with peroxide solutions. For example, U.S. Pat. Nos. 2,119,519 and 2,249,646 disclose the addition of titanium having a valence of less than four to oxidizing bleaching agents, including peroxides. This addition is said to activate the alkaline bleaching process so as to reduce both the treatment time and oxidant concentration needed to obtain a given level of brightness gain. Such reductions advantageously benefit pulp strength since viscosity losses and other pulp degradation are minimized due to the abbreviated oxidant-pulp content. However, even this improved process adduces no significant pulp delignification.
Peracids are one class of oxidizing bleaching agents which effect selective lignin removal. These reagents are essentially derivatives of peroxides. However, they are quite expensive and do not lend themselves to easy recovery and regeneration.